1. Field of the Invention
The present invention relates to a stacked photovoltaic element comprising at least two power-generating function units, and a method for producing the same.
2. Related Background Art
A photovoltaic element is a device which converts incident light energy into electric energy. A solar cell is a photovoltaic element which converts solar rays as white light into electric energy. It is characterized by efficiently converting light over a wide wavelength range into electric energy. It is necessary to efficiently absorb light over a wide wavelength range in order to achieve a high conversion efficiency.
A stacked photovoltaic element, which is formed by stacking a plurality of photovoltaic elements each containing a photoactive layer having a different band gap from each other is known as one of the solutions to achieve a high conversion efficiency. This stacked photovoltaic element has one photovoltaic element having a photoactive layer of larger band gap on a light incident side or thinner photoactive layer, and another photovoltaic element having a semiconductor of smaller band gap or thicker photoactive layer in this order from the light incident side, the former absorbing light of shorter wavelengths and the latter absorbing light of longer wavelengths which the former transmits. The stacked photovoltaic element, therefore, can more efficiently absorb and utilize light over a wider wavelength range.
It is essential to provide each photovoltaic element having a photoactive layer having a different band gap with light of a wavelength in a range suitable for that element, because a wavelength range of incident light which each photovoltaic element can utilize varies depending on the band gap of a semiconductor used as the photoactive layer for that element. In other words, photon cannot be absorbed by the semiconductor when photon has lower energy than the band gap of the semiconductor. In such a case, it only passes through the semiconductor without being utilized. On the other hand, photon having higher energy than the band gap of the semiconductor cannot be fully utilized although it can be absorbed, because potential energy of electron which can be produced when the electron is excited is limited by magnitude of the band gap, whereby the difference between the band gap energy and photon energy cannot be utilized. It is therefore essential to design a stacked photovoltaic element to selectively introduce light of shorter wavelength range into a photovoltaic element on the incident light side and light of longer wavelength range into another photovoltaic element arranged under the above element.
One of the known solutions is to provide an intermediate layer as a light-reflecting layer between these photovoltaic elements. For example, Japanese Patent Application Laid-Open No. S63-77167 and Kenji Yamamoto, “Thin-film polycrystalline silicon solar cell”, Applied Physics, The Japan Society of Applied Physics, May, 2002, Vol. 71, No. 5, p. 524 to 527 disclose a method for providing an electroconductive intermediate layer each between the elements which reflects light of shorter wavelength and transmit light of longer wavelength. Japanese Patent Application Laid-Open No. H2-237172 discloses a method for adjusting the thickness of a selective reflection layer in such a way to increase electric current flowing through a photovoltaic element on the incident light side by setting its peak reflectivity to the maximum wavelength of spectral sensitivity of the photovoltaic element on the incident light side. Japanese Patent Application Laid-Open No. 2001-308354 discloses a method for enhancing efficiency of a stacked photovoltaic element by a selective reflection layer of stacked structure having a higher reflectivity for the shorter wavelength range which the upper photoelectron conversion layer can absorb more easily, and a lower reflectivity for the longer wavelength range which the lower photoelectron conversion layer can absorb more easily for transmission. Each of these techniques uses a dielectric layer of SnO2, ZnO, ITO or the like as the selective reflection layer, to prevent light of short wavelength, which should be absorbed by the photovoltaic element on the incident light side, from being absorbed by the lower photovoltaic element and thereby to enhance conversion efficiency of the photovoltaic element on the incident light side.
As discussed above, the extensive studies on intermediate layers have produced the intermediate layers good to some extent. However, there are problems to be solved to satisfy the demands for improved optical and electrical characteristics, compatibility with a semiconductor layer, and deposition rate.
For example, the following problems occur when the above-described electroconductive reflection layer is provided as the intermediate layer.
A photovoltaic element of large area, comprising unit elements stacked on each other in series, e.g., as shown in FIG. 1, has electrical defects within the element, resulting from dust generated during the deposition step, or irregularities or foreign matter on the surface of a substrate. In FIG. 1, numeral 100 denotes the stacked photovoltaic element; 101: substrate, 102: second photovoltaic element, 103: zinc oxide layer, 104: first photovoltaic element, 105: electroconductive layer as transparent electrode, 106: short circuit in the second photovoltaic element, and 107: short circuit in the first photovoltaic element. The electrical defects inevitably associated with the large area deteriorate the element characteristics resulting from decreased shunt resistance and fill factor (FF). One of the effective means to solve these problems is a method (passivation) in which a photovoltaic element is normally dipped in an electrolytic solution and an electric current is passed through it to selectively remove a part of an electroconductive layer outside of an electric defect. However, for a stacked photovoltaic element comprising a lower photovoltaic element layer, an intermediate layer, an upper photovoltaic element layer and an electroconductive layer on a substrate, the above procedure can partly remove the electroconductive layer 105 on the first photovoltaic element layer as the upper layer containing the defect 107, but cannot remove the intermediate layer 103 on the second photovoltaic element layer as the lower layer containing the defect 106. As a result, short-circuit current flows through the defect in the lower layer to decrease electromotive force of the lower photovoltaic element. Short-circuit current cannot be effectively prevented from spreading into the intermediate layer for various reasons. In particular the intermediate layer should have a certain thickness to function as the reflection layer, and should satisfy the other considerations, e.g., compatibility with the semiconductor layers with which it is in contact on both sides and series resistance. These requirements limit a range in which the resistivity of the materials can be adjusted. Moreover, additional junctions generated between the photovoltaic elements and an intermediate layer of a different material therebetween inevitably deteriorate characteristics which are accompanied by decreased FF. Still more, a plurality of layers inserted to prevent a reduced shunt resistance further increase junction number, thus further aggravating the interfacial problems.
As discussed above, incorporation of an intermediate layer as a selective reflection layer for increasing photocurrent involves the adverse effect of decreased electromotive force of the photovoltaic element.
Moreover, the method disclosed by Patent Document 3, although giving a selective reflection layer structure satisfying the light reflection and light transmission characteristics, is found to be still insufficient in producing the intermediate layer having no adverse effect on the photovoltaic element and having good connection with the photovoltaic element. For the intermediate layer to have sufficient characteristics, the following technical requirements should be satisfied.
The intermediate layer should be kept in good ohmic contact with the semiconductor layer under the intermediate layer. It should be formed in such a way to cause little damages to the underlaying semiconductor layer by chemical modification (e.g., by oxidation) or physical modification (e.g., by ion-caused damages). Moreover, it should have an adequate resistivity and film thickness, and should be designed to control lateral flow of shunt current via the intermediate layer.